When a buried electrical cable suffers a fault, such as a partial break or rupture, of its cable sheathing or jacket, electrical current will “leak” out of the cable through the cable sheathing fault into the ground. It is thus necessary to repair such a fault, for which the fault must be located and then exposed for access and repair by excavating the ground at a proper location at and around the fault location. Various techniques exist for electrically locating such a cable sheathing fault, to exactly localize the position of the fault, so that the smallest possible area of the cable must be dug-out or excavated.
For example, to carry out such a cable sheathing fault location, it is known to use an electrical signal generator or transmitter to apply a pulsed DC voltage between the cable shielding conductor and the ground potential. As a result, current will flow from the cable shielding conductor through the cable sheath or jacket fault at the location of the fault out into the ground, and then through the ground back to the ground potential connection of the electrical signal transmitter. Due to the electrical resistance of the ground, and the current flowing through it, a voltage funnel is formed in the ground at the cable sheathing fault location. This produces a characteristic voltage pattern emanating through the ground from and around the cable fault location. Thus, using a measuring device with ground probes such as ground spikes that are driven into the ground for achieving a galvanic coupling, it is possible to measure this voltage pattern or voltage signal in the ground. As one approaches the fault location above the cable, the voltage that is measurable at the ground surface increases toward the voltage source, i.e. the fault location. Then, very close to the location above the cable fault the voltage drops off and becomes zero directly above the fault location. Then, moving along the cable after the fault location, i.e. in the direction opposite and away from the voltage signal transmitter connection, the voltage switches polarity, e.g. becomes negative.
In the above fault locating method, the problem often arises that interference voltages are additively superimposed on the useful signal that is to be measured, and thereby impair the measurement. For example, especially problematic in that regard are exponentially decaying transient interferences, which arise, for example, when sticking the ground probes or ground spikes into the ground. Such a transient interfering voltage signal can be many times greater in amplitude than the pulsed DC voltage that is to be measured as a useful signal. Thereby, the time constant of the interference spikes or pulses is typically significantly greater than the period of the pulsed DC voltage. Thus, measurements made during the signal decay time are not valid. According to the current state of the art, this problem is addressed by two methods.
A first known method involves applying a high pass filtering, for example in the simplest case an RC high pass filter. The time constant of such a high pass filter is typically significantly smaller or shorter than the period of the pulsed DC voltage. As a result, the RC high pass filter performs as a differentiator. Thus, if the pulsed DC useful signal is a square or rectangular wave signal, this leads to a voltage reversal or alternation at the times of the flanks of the rectangular wave signal. This alternation or reversal makes it difficult or even impossible to accurately recognize the polarity of the voltage signal in the ground being measured. In turn, this means that it is difficult or impossible to accurately identify the direction of the fault location relative to the measurement location.
On the other hand, according to a second method, the user can himself regulate out the applicable offset voltage. In this case the polarity of the measured voltage, and thus the direction of the cable sheathing fault, can be recognized. However, such manual “regulating-out” of the offset voltage is not very easy, and thus requires great experience and/or skill of the user. Furthermore, especially the abovementioned transients that arise when the ground probe or spike is stuck into the ground make this type of manual regulation difficult, because the user either must constantly re-regulate the equipment to regulate-out the offset voltage or must wait until the probe insertion transients have decayed, which can take several minutes. Because a great number of measurements may be necessary for the final locating of the cable sheathing fault, such waiting for the probe insertion transients to decay after each new insertion of the ground probe leads to a measurement process that is very time consuming overall.